A Complete Guide to Surface Finish of CNC Machined Parts

1 Introduction:

In recent years, CNC machining technology has development rapidly, surface finish has become a major content of engineering design. In mechanical processing, through certain processing of the surface of the materials or products, which can not only improve the capability and quality, such as corrosion resistance, wear resistance, oxidation resistance, etc., to ensure the safety and reliability of the product, extended service life, but also can beautify the appearance, and improve the added value of the product and market competitiveness. Surface finishing technology has been widely used in CNC Machined parts, which plays an important role in improving the performance, quality and appearance of construction CNC machined products.

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2 Definition of Surface Finish

Surface finish is to design the material surface and matrix as a system, using surface modification technology, film technology and coating technology, so that the material surface to obtain the material itself does not have the desired performance of the system engineering. It includes plating and technology, coating and testing equipment, surface composition and structure analysis technology, surface performance testing technology, testing methods, standard evaluation, quality assurance and process control for the manufacture of new surface and surface layer to form a complete set of engineering, large-scale technology. In a word, surface finish is the technique of obtaining a surface layer on the surface of a material or product that differs in performance from the substrate itself.

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3 Functions of Surface Finish for CNC Machined Parts

CNC machined surface finishing changes the morphology, chemical composition and organizational structure of the surface and near surface area of materials in the most economical and effective way, or endows materials with a new surface. Its role can be summarized as follows:

Adapt to the development of production, improve product quality and economic benefits.

(2)Provide special materials for other High-Tech.

(3)Provide effective means for the technical upgrading and maintenance of device and equipment.

(4)Energy and resources saving.

(5)Decorative and aesthetic.

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4 The Role of Surface Finishing in Construction Machinery

Rust Protection

Many parts and components of CNC machine are metal materials,  the equipment is basically in the open air, and the exposed metal is easy to be corroded. Therefore, the CNC machined parts, the manufacture and the whole machine need certain anti-rust protection, and apply electroplating, coating and other surface finish technology. Block metal direct contact with air and water is the prerequisite to prevent the metal corrosion, and plating technology in surface finishing is in CNC machined parts or form a layer on the surface of the whole machine firmly attached to the metal coating on the surface of the metal parts or organic polymer coating on the isolation of water and air, which have the effect of rust protection.

Exterior Decoration

With the continuous development of CNC machine technology and the improvement of users’ requirements for products, the appearance of products has been improved, and has become one of the main factors to market competition. On the one hand, the advance of the appearance lies in the improvement of the overall design of the product, on the other hand, lies in the application of advanced surface finishing technology. Electroplating and coating not only play a protective role in construction machinery equipment, but also play a very obvious and important decorative role, giving products a rich and colorful appearance, making construction machined parts more pleasant in color and tone.

Surface Strengthening

CNC machine has complex structure, some parts not only requires enough stiffness and toughness, but also requires enough hardness and wear resistance on the surface, such as gear tooth surface, transmission shaft surface, etc., are required to have higher surface hardness and better wear resistance. Surface heat treatment is the most commonly used process technique, and shot blasting is used for surface strengthening of metal parts.

Part Repair

Surface finishing technology not only plays roles in the manufacturing process of CNC machined parts, but also can repair damaged parts and remanufacture discarded surface finish in the process of using construction machinery, extend the service life of CNC machine parts, improve its utilization value and reduce resource consumption.

 

5 Current Status of Surface Finish Technology on the Parts

Surface Spray Technology

The surface spraying technology applied in mechanical parts processing industry is mainly the coating technology of organic polymer coating, the application of coating technology runs through the whole process of mechanical manufacturing. After the raw materials into the factory, need to be pretreated, spraying weldable primer or shop primer, and in the premise the steel cutting, welding performance not being effected, to avoid steel corrosion in storage and processing process. After welding into parts, it is necessary to paint anti-rust primer to prevent the corrosion of subsequent processing, storage and assembly process. After the assembly of parts or the assembly of the whole machine, it is necessary to paint parts and the whole machine, and spray all kinds of finishing coat to obtain a good product appearance. At present, many construction machinery enterprises are promoting parts finishing coat, that is, when parts are coated, not only anti-rust primer is sprayed, but also topcoat is painted according to the quality requirements of the whole machine. After product assembly, the whole machine is no longer coated, but only local paint repair. Mechanical parts coating process varies greatly according to different plate thickness, generally divided into structural parts coating and thin plate parts coating. Structural parts are generally treated by shot blasting and then sprayed with solvent-based coating. Structural parts coating equipment mainly includes shot blasting cleaning machine, paint room, levelling room, drying room, air conditioning ventilation system, ventilation and dust removal system, workpiece conveying system, etc. The specific parameters of coating equipment are customized according to the production parts, different manufacturers are different. Thin plate parts are coated with phosphating pretreatment, and then coated with solvent based coating or electrophoretic coating or powder coating. The coating of thin plate parts of construction machinery is generally composed of pickling, phosphating, painting, levelling, drying, air supply and exhaust systems of air conditioning, and the specific parameters are customized according to the parts produced. Due to the characteristics of the industry and products, the overall technical level of construction machinery painting is not high, mainly with traditional solvent based coating, primer generally adopts high-pressure airless spraying or mixed air spraying, and finishing coat generally adopts mixed air spraying and air spraying. In recent years, the construction machinery industry continues to develop rapidly, the production scale growing, and the technology level has been greatly improved, so as to electrophoresis coating, powder coating gradually popularized and applied. At present, the domestic construction machinery industry has a number of professional electrophoretic coating production lines and powder coating production lines, mainly used in cab, machine cover and other thin plate parts. Cathodic electrophoresis is used in most cases, electrostatic spraying is used in powder coating.In general, electrophoretic coatings are used as primer coatings, and powder coatings are used in conjunction with electrophoretic primers as finishing coatings. Electrophoretic coating and powder coating are not only environmentally friendly, but also have better comprehensive performance than traditional solvent based coating. For example, electrophoretic primer has no dead angle, so it has been paid more and more attention in the CNC industry and has become the key choice of construction machinery new assembly line.  The main reasons that affect the large-scale promotion of electrophoretic coating and powder coating in construction machinery are as follows: construction machinery structural parts are mainly thick plate parts with large size, resulting in high energy consumption cost of coating drying and huge equipment investment. If a breakthrough can be made in coating drying process and specialized coating production can be realized in production scale, it will promote the progress of coating technology and the widespread application of electrophoresis coating and powder coating in CNC  parts processing industry.

Following shows the typical coating process of construction machinery cover.

NO

Processing name P.S

1

Pre-cleam By hand
2 Pre-degrease

Coating

3

Degrease

Coating-dip-coating

4 Wash

Coating

5

Surface adjust

Coating-dip-coating

6

Phosphating

Dip-coating

7

Wash

Coating

8

Circle DI wash

Coating

9

Clean DI wash

Coating

10

Drain

11

Phosphating Test

12

Electrophoresis

13

Cathode electrophoresis

Dip+1 UF liquid spray

14

1 UF liquid spray

Coat+2 UF liquid spray

15

1 UF liquid spray

Dip+3 UF liquid spray

16

1 UF liquid spray

Coating

17

Clean UF liquid wash

Coating

18

Circle DI wash

Dip

19

Clean DI wash

Coating

20

Drain/rotating

21

Electrophoresis drying

Heated air circulationn

22

Strong cold

23

Polish

By hand

24

Cleaning

By hand

25

Powder coating

Auto/hand

26

Drying

Heated air circulationn

27

Strong cold

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Chemical Processing Technology

Chemical surface finish technology is widely used in various parts of construction machinery, mainly as the pretreatment of coating, anti-rust pretreatment and final anti-rust, decoration treatment. Coating pretreatment mainly uses phosphating, phosphating film generated on the surface of iron and steel parts can improve the adhesion between the coating and the workpiece surface, so it becomes the main pretreatment process of coating, especially for electrophoretic coating, phosphating pretreatment is essential. In addition, shot blasting is easy to cause deformation of thin plate parts, so phosphating can only be used.With the increase of the demand for construction machinery and the expansion of production capacity, the construction machinery industry has built a number of professional phosphating production lines with high level in recent years, which has effectively promoted the improvement of the coating quality of construction machinery. Phosphating waste liquid has certain pollution to the environment, the cost of environmental protection treatment is also high, so many enterprises are actively exploring phosphorus free and other environmental protection phosphating alternative technology.

In construction machinery, phosphating not only as a coating pretreatment, but also as a rust pretreatment process,which is widely used. The pipe conduit of the hydraulic system has rubber hose and metal hard tube. In order to ensure the cleanliness of the hydraulic system, there is no rust and debris in the hard tube, so it is necessary to use phosphating treatment. After removing the rust in the pickling tube for phosphating, so that the inner wall of the hard tube to generate a porous phosphating film, and then coated with oil for protection, to avoid the hard tube inner wall rust again and affect the hydraulic system. In addition to phosphating, electroplating and blackening treatment are also used for chemical treatment of rust prevention. The application of blackening treatment is seldom use in construction machinery, mainly some bolts, nuts and other standard parts and hydraulic valves, but electroplating technology is more applied. Bolts, nuts, washers and other standard parts and hydraulic hose joints, valve seats and other hydraulic accessories are widely used in electroplating technology and zinc plating, so as to prevent these parts being corroded in the production and use. Another purpose of electroplating is to improve the surface wear resistance, such as hydraulic system cylinder piston rod, pin shaft, sleeve, etc., hydraulic cylinder piston rod surface, which is generally treated with chrome plating. The appearance of the metal finishing is bright and smooth, which not only protects the product from rust, but also has a good decorative effect on the surface of the product. Electroplating is also widely used in maintenance and remanufacturing of construction machinery. Some parts such as pin shaft, due to the long time work has been serious wear and tear, the cost of replacing the new pin shaft is higher and the processing and manufacturing cycle is longer, and the old pin shaft is repaired by brush plating technology, so that the size of its appearance is up to its design requirements, not only the production cycle is shortened, and the cost is reduced.

The promotion of CNC machined processing has provided a wider application for electroplating. If the wastewater produced by electroplating is not treated, it will cause serious pollution to the environment. Construction machinery production enterprises generally do not have electroplating production workshop, electroplating parts are generally outsourcing, or the electroplating process for external cooperation, by professional electroplating enterprises to undertake the production of electroplating parts or electroplating process. In fact, electroplating technology is also developing, some environmentally friendly and with good overall performance of electroplating process should also be promoted in the CNC machined processing.

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Surface Strengthening Technology

The surface strengthening technology used in CNC manufacturing is mainly shot blasting strengthening technology and surface heat treatment technology, which used for the surface strengthening of gear, shaft sleeve transmission parts, improve hardness and wear resistance.

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Surface Shot Blasting Technology

Shot blasting is a surface deformation strengthening technology, which can remove surface rust and oxide, improve the fatigue resistance of mechanical products and metal parts, eliminate stress concentration, extend its service life, and optimize the surface process state of the workpiece. Therefore, it has been widely used in the CNC machined processing industry. In CNC machine tools, shot blasting is not only used as the surface strengthening technology of gear and pin shaft parts, but also as the coating pretreatment technology of structural parts. Conical spline gear is one of the key parts of CNC machined processing transmission system, with high dimensional accuracy and complex manufacturing process. The manufacturing process covers more than ten processes of forging, heat treatment, mechanical processing, shot blasting 4 kinds of processing technology, including two shot blasting after quenching and tempering treatment and tooth surface heat treatment, the last shot blasting is to strengthen the surface and eliminate stress. The equipment of shot blasting strengthening process generally adopts efficient automatic shot blasting cleaning machine. In the actual production, the choice of shot blasting cleaning machine should be customized according to the specific parts size. In order to achieve the purpose of shot blasting strengthening, it is necessary to choose proper shot and reasonable shot blasting time. Shot blasting treatment commonly used shot casting steel shot, steel cut shot, alloy shot, etc., the diameter of cast steel shot is generally between 0.8 ~ 1.2mm, low hardness and easy to be broken, short life. Steel wire cutting shot is made of mine waste steel wire rope to cut, degreasing, with sharp corners and a good surface cleaning effect. But for the purpose of strengthening the surface of the shot blasting process is not suitable for direct use, should be pretreated steel wire cutting shot, remove sharp corners; Due to the high cost of wire cutting shot pretreatment, standard alloy shot blasting is generally used for strengthening purposes.

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Surface Heat Treatment Technique

Heat treatment is an essential process for rotating parts and connecting pins of CNC machine by reform the structure of materials to meet its performance. The production process of cone spline gear goes through three different heat treatments, the last of which is carburizing and quenching to improve the hardness and wear resistance of the surface. According to the application requirements of different parts in construction machinery, the surface heat treatment strengthening technology is also different, including carburizing, nitriding, quenching, etc., and the commonly used quenching process is high-frequency surface quenching, laser surface quenching, etc. The number and type of workpiece for heat treatment are more in the processing of mechanical parts, but the purpose and technical requirements are not the same. In actual production, the appropriate heat treatment process should be selected according to the purpose, material and surface hardness requirements of the workpiece. Whether carburizing, nitriding, or high-frequency quenching, laser surface quenching, induction quenching technology, heat treatment for CNC machine tools, is a core technology, the choice of equipment and process parameters is very key, the result of heat treatment will determine the performance and service life of key components of construction machinery and the whole machine. For example, the transmission gear of engineering machinery, with complex working conditions and large load bearing, which requires the gear to have high fatigue strength, requires the workpiece to have a certain toughness inside, and requires the surface to have a certain hardness and wear resistance. This kind of gear is made of 20CrMnTi material. In order to meet the design and use requirements of parts, it is generally required to carry out gas carburizing first, and then fast heating quenching and tempering, so that the surface hardness reaches HRC60, the root hardening depth is above 1.5mm, and the tooth surface hardening depth is between 2.2 and 2.8mm. And the internal structure only needs quenching and tempering treatment. The hardness is between HB269 and 381.

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6 Factors to Consider When Choosing a CNC Machining Surface Finish

CNC finishing is a key post-process method that essentially determines the look, feel, and function of a part. It is essential to know you can choose the best finish for your part. Here are some of the factors to consider to make sure you choose the right one from the start:

The Product’s Application

Different parts are subjected to different environmental conditions, be it vibrations, heat, moisture, UV light, etc. Careful consideration of who and what the product is for will help you make the right choice. Metals intended for outdoor use and increased exposure need a standard machining surface finish that improves their hardness. The chosen finish should also increase the wear and metal resistance. Parts used in construction will place less focus on aesthetics. Durability is often the major concern in this case.

Durability Concerns

You need to ask yourself how long you want your product to last. Durability is a major part of manufacturing. Although the raw material plays a crucial role here, you cannot leave out the machining surface finish. Adding value to your finished product involves durability, and you should choose the right finish for that.

Relationship Between the Raw Material and the Metal Surface Finish

Some materials are hard to polish. It may be challenging to anodize some other surfaces, while paint may not stick well to certain materials. Therefore, you must consider CNC finishing from the beginning of the design process. Do not wait till the last minute to decide the surface finish for your CNC machined part. Research to ensure that your surface finish matches the raw materials involved.

The Part’s Dimensions

One important point to note is that the machining surface finish may change a part’s dimensions. Thick finishes such as powder coating may increase the surface thickness of the metal material. Coatings may also affect the mating surfaces and clearances of parts that must fit. Finishes that use chemicals may eat away a tiny degree of the material and slightly affect dimensions. You must calculate these factors at the beginning of the project to ensure quality production.

Cost

Having considered the above-listed factors, you should then consider your budget for the project. The best finishes often need careful preparations and complex procedures. They also require the use of higher-quality materials. All of these are cost drivers when it comes to manufacturing. So, you need to consider these and compare them with your budget before choosing a machining surface finish.

7 Conclusion:

CNC machine tool surface treatment technology for construction machinery, has a very important significance, from the surface treatment of parts to the whole machine coating decoration, the production and manufacture of construction machinery can not leave the surface finishing technology.CNC machined parts are an important direction of the development of construction machinery in the future, and will be widely used in surface finishing technology. It should be pointed out that surface finish technology is a marginal science. Whether it is coating or electroplating, it is related to chemical engineering, and backward technology will cause environmental pollution.Therefore, construction machinery in the selection of surface finish technology, to actively choose environmental protection of the process and equipment, and to increase the input of the existing equipment process, minimize the pollution of the environment. CNC machine tool surface treatment technology for construction machinery, has a very important significance, from the surface treatment of parts to the whole machine coating decoration, the production and manufacture of construction machinery can not leave the surface finish technology.

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